ISL6277HRZ datasheet(32/37 Pages) RENESAS

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ISL6277

FN8270 Rev 1.00

Page 32 of 37

Mar 8, 2012

remove the switching ripple of the phase node voltages. It is

recommended to use a rather long RisenCisen time constant such

that the ISEN voltages have minimal ripple and represent the DC

current flowing through the inductors. Recommended values are

Rs = 10kΩ and Cs =0.22µF.

Thermal Monitor Component Selection

The ISL6277 features two pins, NTC and NTC_NB, which are used

to monitor motherboard temperature and alert the AMD CPU if a

thermal issues arises. The basic function of this circuitry is

outlined in the Thermal Monitor section. Figure 32 shows the

basic configuration of the NTC resistor, RNTC, and offset resistor,

RS, used to generate the warning and shutdown voltages at the

NTC pin.

As the board temperature rises, the NTC thermistor resistance

decreases and the voltage at the NTC pin drops. When the

voltage on the NTC pin drops below the thermal warning

threshold of 0.640V, then VR_HOT_L is pulled low. When the

AMD CPU detects VR_HOT_L has gone low, it will begin throttling

back load current on both outputs to reduce the board

temperature.

If the board temperature continues to rise, the NTC thermistor

resistance will drop further and the voltage at the NTC pin could

drop below the thermal shutdown threshold of 0.580V. Once this

threshold is reached, the ISL6277 shuts down both Core and

Northbridge VRs indicating a thermal fault has occurred prior to

the thermal fault counter triggering a fault.

Selection of the NTC thermistor can vary depending on how the

resistor network is configured. The equivalent resistance at the

typical thermal warning threshold voltage of 0.64V is defined in

Equation 37.

The equivalent resistance at the typical thermal shutdown

threshold voltage of 0.58V required to shutdown both outputs is

defined in Equation 38.

The NTC thermistor value correlates to the resistance change

between the warning and shutdown thresholds and the required

temperature change. If the warning level is designed to occur at a

board temperature of +100°C and the thermal shutdown level at

a board temperature of +105°C, then the resistance change of

the thermistor can be calculated. For example, a Panasonic NTC

thermistor with B = 4700 has a resistance ratio of 0.03939 of its

nominal value at +100°C and 0.03308 of its nominal value at

+105°C. Taking the required resistance change between the

thermal warning threshold and the shutdown threshold and

dividing it by the change in resistance ratio of the NTC thermistor

at the two temperatures of interest, the required resistance of

the NTC is defined in Equation 39.

The closest standard thermistor to the value calculated with

B = 4700 is 330kΩ. The NTC thermistor part number is

ERTJ0EV334J. The actual resistance change of this standard

thermistor value between the warning threshold and the

shutdown threshold is calculated in Equation 40.

Since the NTC thermistor resistance at +105°C is less than the

required resistance from Equation 38, additional resistance in

series with the thermistor is required to make up the difference.

A standard resistor, 1% tolerance, added in series with the

thermistor will increase the voltage seen at the NTC pin. The

additional resistance required is calculated in Equation 41.

The closest, standard 1% tolerance resistor is 8.45kΩ.

The NTC thermistor is placed in a hot spot on the board, typically

near the upper MOSFET of Channel 1 of the respective output.

The standard resistor is placed next to the controller.

Layout Guidelines

PCB Layout Considerations

POWER AND SIGNAL LAYERS PLACEMENT ON THE PCB

As a general rule, power layers should be close together, either

on the top or bottom of the board, with the weak analog or logic

signal layers on the opposite side of the board. The ground-plane

layer should be adjacent to the signal layer to provide shielding.

COMPONENT PLACEMENT

There are two sets of critical components in a DC/DC converter;

the power components and the small signal components. The

power components are the most critical because they switch

large amount of energy. The small signal components connect to

sensitive nodes or supply critical bypassing current and signal

coupling.

The power components should be placed first and these include

MOSFETs, input and output capacitors, and the inductor. It is

important to have a symmetrical layout for each power train,

preferably with the controller located equidistant from each

NTC

RNTC

30µA

INTERNAL TO

ISL6277

FIGURE 32. THERMAL MONITOR FEATURE OF THE ISL6277

MONITOR

Warning

640mV

Shutdown

580mV

VR_HOT_L

330k



8.45k



0.64V

A

----------------

21.3k



(EQ. 37)

0.58V

A

----------------

19.3k



(EQ. 38)

21.3k

 19.3k

–



0.03939 0.03308

–



------------------------------------------------------

317k



(EQ. 39)

330k

 0.03939





330k

 0.03308





–

2.082k



(EQ. 40)

19.3k

 10.916k

–

8.384k



(EQ. 41)

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